Strength of the H 2 O near‐infrared absorption bands in hydrated minerals: Effects of particle size and correlation with albedo

We use laboratory experiments and radiative transfer modeling to study how the 1.9‐ and 3‐ μ m H 2 O absorption bands are affected by variations of mineral particle size and albedo. A consistency between results of physical experiments and numerical simulations demonstrates that the studied effects are well described by the theory of radiative transfer in particulate media. Band strengths show different relationships with particle size, depending on the absolute intensity of the absorption and the criterion used to calculate band strength (band depth, area, ESPAT function, etc.). Various mixing processes used to vary sample albedo reveal a strong dependence of band strengths with albedo. For the 1.9‐ μ m band, those effects result in variations of the hydrated minerals detection limit by more than 1 order of magnitude. However, the shape of the relationship between the 1.9‐ μ m band strength and albedo could be used to get information on the mixing mode (intimate, granular, or geographic) between hydrated and nonhydrated minerals. For the 3‐ μ m band, we found a strong linear correlation between the integrated band area and the continuum reflectance that opens a promising way to isolate the effect of albedo on planetary surfaces and retrieve spatial variations of material hydration state. When this spectral criterion is used for the 3‐ μ m band, the effects of particle size are very limited for particles larger than 150 μ m but remain important below this value. Therefore, an independent way to derive particle size from remote sensing appears necessary to address the effects of particle size variations where small particles are present.